The storage capacity of present optical memory devices appear quite inadequate for many applications. One does not know how to do more than storing information data at the surface of a photo-sensitive layer covering the disc side. Most often, micro-dips on which the light beam is focused are engraved on such an optical coating, which form elementary cells for data recording that are distributed over the photo-sensitive surface. Owing to the area each of them occupies and to their necessary spacing, their number is limited.
With a view to increasing the memory capacity of optical discs, several solutions are currently been studied. One may mention:--double-sided discs,--the use of blue laser light, allowing a slight size reduction for the elementary cells in agreement with the theory of diffraction,--the superposition of several photo-sensitive layers, in limited number however (six for example or up to ten), which in addition requires as many lenses to focus the light on the different layers, both for recording and read-out.
These different solutions allow only to increase the capacity of current optical discs by at most ten times, which is not a sufficient increase for the needs of industry.
One can also envisage to use substrates bearing several differently coloured layers, as is the case in color photographic films. But, in fact, it is difficult to have more than a few layers, owing to the interactions among the coloured light beams to be used both for recording and read-out, and therefore the memory increase remains limited.